1. Field of the Invention
The present invention relates to a wireless medium access control (MAC) protocol, and more particularly relates to a hybrid wireless MAC protocol which uses a Ready To Send(RTS)/Clear To Send(CTS) exchange during a contention free period (CFP) in order to avoid contention from Stations (STAs) in overlapping basic service sets (BSSs).
2. Description of Related Art
The wireless communication market has lately enjoyed tremendous growth and is now capable of reaching every place on earth. Hundreds of millions of people exchange information every day using pagers, cellular telephones and other wireless communication products. Wireless communication has broken the harnesses of wireline networks, allowing users to access and share information on a global scale nearly everywhere they venture.
Standard LAN protocols (wireline), such as ETHERNET™, operate on wireline networks using various MAC protocols, e.g., carrier sense multiple access with collision detection (CSMA/CD), at fairly high speeds with inexpensive connection hardware which provides an ability to bring digital networking to practically any computer. Until recently, however, LANs were limited to physical, hard-wired (wireline) infrastructure. Even with phone dial-ups, network nodes were limited to access through wireline connections. Wireline communications, however, have set the stage for wireless communications.
Since the recent development of wireless LANs, many network users, such as mobile users in business, the medical professions, industry, universities, etc., have benefited from the enhanced communication ability of wireless LANs, i.e., increased mobility. Uses for wireless network access are practically unlimited. In addition to increased mobility, wireless LANs offer increased flexibility. Compared to wireline counterparts, however, wireless networks are known to have much less bandwidth, and hence it is highly desirable to utilize the wireless link bandwidth efficiently.
To that end, commonly owned pending application Ser. No. 09/732,585, filed Dec. 8, 2000, and entitled: A Wireless MAC Protocol Based On A Hybrid Combination Of Slot Allocation, Token Passing and Polling For Isochronous Traffic, discloses a mechanism for increasing the efficiency of bandwidth use. The '585 application, incorporated herein by reference, utilizes a hybrid MAC protocol with a combination of bandwidth allocation, a variation on conventional token passing and polling to regulate isochronous traffic efficiently within a wireless network with “hidden” terminals.
The IEEE standard for wireless LAN protocol is identified as “Standard for Information Technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific Requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY),” 1999, which will be referred to hereinafter as IEEE 802.11. IEEE 802.11 specifies parameters of both the physical (PHY) and medium access control (MAC) layers of the network. The PHY network may handle transmission of data between nodes by either direct sequence spread spectrum (DSSS)/complementary code keying (CCK) supporting 1–11 Mbps, frequency-hopping spread spectrum (FHSS) supporting 1 or 2 Mbps, infrared (IR) pulse position modulation supporting 1 or 2 Mbps, or orthogonal frequency division multiplexing (OFDM) modulation supporting 6–54 Mbps.
The MAC layer is a set of protocols which is responsible for maintaining order in the use of a shared medium. IEEE 802.11 specifies a carrier sense multiple access with collision avoidance (CSMA/CA) protocol for use as a random access protocol technique. A CSMA protocol operates as follows. A station (STA) senses the medium, which, if busy, defers transmission of its data packet to a later time. A problem will arise in the case where two (2) STAs sense the medium as being free, for example, contemporaneously, and each transmit a data packet at the same time resulting in a collision. Note that in wireless environment, transmitting and receiving at the same time is almost impossible even with a full duplex radio due to the high signal attenuation. That is, if one senses the medium while it transmits a packet, it will only sense its own packet even if the packet is colliding with another packet in the medium.
Moreover, in such a wireless LAN system, not all STAs can “hear” each other. The 802.11 standard includes collision avoidance (CA) mechanism in order to minimize collisions, which could arise from two STAs, transmitting at the same time.
The conventional mechanism attempts to overcome the problem by implementing the following rules. 1. If a station wishing to transmit a data packet senses that the medium is busy, it defers its transmission. If the station “listens” for a random length of time and finds the medium free, the STA will then transmit. As the reader can guess, this is certainly not a complete solution to the above-stated problem. 2. Alternatively, the receiving station implements a cycle redundancy check (CRC) of the received packet and sends an acknowledgment packet (ACK) to the transmitting station, indicating to the transmitting STA that no collision has occurred. If the transmitting station does not receive the ACK, it retransmits its data packet until it actually receives the ACK, or discards the data. As with rule 1., this is not a complete solution.
Moreover, radio transmissions based on IEEE 802.11 may also be ineffective because transmitting nodes within the wireless LAN cannot hear any other node in the system (network) which may be transmitting. That is, the transmitting node's own signal is presumably stronger than any other signal arriving at the node. The problem can be analogized to the problem of hearing impairment, that is, some nodes are hearing impaired for any of various reasons.
Hidden nodes or stations (STAs) prevent efficient use of bandwidth as a result of their hearing impairment to certain transmissions. For example, FIG. 1 shows a conventional wireless local area network (WLAN) composed of an access point (AP) and a number of stations (STAs). WLAN operation therein is based on the premise that the AP can communicate with all STAs directly over the wireless link while STAs can communicate each other depending on the relative locations due to their limited transmission ranges.
In order to reduce the probability of two STAs transmitting data which will collide because the STAs are not aware of the other's presence (can not “hear” each other) defines a virtual carrier sense (VCS) mechanism. The STA to transmit sends a short control packet referred to as a request to send(RTS) packet. The RTS includes identification of the STA source, its destination, and the duration of its data packet transmission time and time for receipt of the ACK packet. The destination STA responds with a clear to send(CTS) if the medium is free, also including time duration information. All STAs receiving either the RTS or CTS packets set their virtual carrier sense indicators, referred to as network allocation vectors (NAV), for the given time period, and utilize same with their physical carrier sensing mechanism when sensing the medium (see FIG. 2). This reduces the probability of collision.
In prior art FIG. 1, STA 1 is seen as clearly able to communicate by its access point AP1 with STA 2 by its access point AP2, either directly or in one hop, but not with STA 3 and its access point AP3. In FIG. 1, a circle around each STA (and access point A) represents the corresponding transmission range, where STAs 1 and 3 are called hidden terminals to each other since they cannot know even the existence of each other without the help of the access point A in between. Note that the communication between STAs 1 and 3 should be performed via the access point A.
The IEEE 802.11 MAC sub-layer defines two functions for accessing the wireless medium: distributed coordination function (DCF) and point coordination function (PCF), as seen in FIG. 2. The DCF is used to transmit asynchronous data based on Carrier Sense Medium Access with Collision Avoidance (CSMA/CA) mechanism, while the PCF uses a polling mechanism for a “nearly isochronous” service.
The PCF is implemented on top of the DCF, and controlled by a Point Coordinator (PC) which resides inside the access point (AP). An example of the PCF access is shown in FIG. 3. The transmission time is divided into super-frames, where each super-frame is composed of a Contention Free Period (CFP) and a Contention Period (CP). During the CFP, the PCF is used for accessing the medium, while the DCF is used during the CP. The duration of a super-frame is referred to as Contention Free Period Repetition Interval (CFPRI). A CFP starts by a beacon frame sent by the AP or PC. A CFP starts with a beacon frame and finishes with a CF-End frame, both transmitted by the AP (See FIG. 3). The beacon includes the information about the real duration of the CFP to update the network allocation vector (NAV) of the STAs as well as the network synchronization information. A Target Beacon Transmission Time (TBTT) indicates the time when the AP attempts to transmit a beacon, so TBTTs repeat every beacon period. A CFPRI is composed of a number of beacon periods. In some situations, the transmission of the beacon frame can be delayed if a DCF frame from the previous repetition interval carries over into the current interval. This situation is known as stretching, and can be seen in FIG. 2 as ‘Delay (due to a busy medium)’. During the CFP, there is no competition for the medium. The AP polls each STA asking for pending frames to be transmitted. In case the STA has any, it will transmit a frame. If the AP receives no response from a polled STA after waiting for a point inter-frame space (PIFS) interval (FIG. 3), it will poll the next STA.
FIG. 4 highlights a situation of overlapping among two BSSs in order to use it for the presentation of our invention. For example, a circle around each STA (and AP) which represents the transmission range of the STA. STAx,1 belongs to the BSS of APx, which is called BSSx. The APs can always reach all the STAs belonging to its BSS, and therefore, all the STAs can always reach its own AP. Unless stated otherwise, the effects of the overlapping STAs will be considered with respect to STA1,1 belonging to BSS1. In the overlapping BSS situation of FIG. 4, AP1 can hear STA1,1 and STA1,2 (BSS1); (2) STA1,1 can hear AP1, STA1,2, and STA2,1; and (3) STA2,1 can hear AP1, STA1,1, and STA2,1. Then, in BSS2, (1) AP2 can hear STA2,1; and (2) STA2,1 can hear AP2, STA1,1, and STA1,2. This can happen, for example, in a block of offices, where the BSSs located in two neighboring offices, apartments, etc., interfere to each other.
Here, the main concern is the performance of the CFP under PCF in BSS1 in the existence of the overlapping BSS2. For example, the transmission from STA1,1 to AP1 during a CFP can collide with the transmission from STA2,1 to AP2. This kind of collision during the CFP can result in severe degradation of the effectiveness of the PCF in terms of the throughput, and it makes really difficult to support QoS using this polling-based PCF.